1. Field of the Invention
The present invention relates to communication optical systems used for a communication apparatus such as a free-space optics communication apparatus, which project light from a light source to another apparatus and guide the light from the receiving apparatus to a light-receiving element.
2. Description of Related Art
FIG. 5 shows the structure of a communication optical system of a free-space optics communication apparatus, which is disclosed in Japanese Patent No. 3339014.
In FIG. 5, reference numeral 1 denotes a light source, reference numerals 2 and 6 denote light-receiving elements, and reference numeral 110 denotes a first prism. The prism 110 includes a beam-splitting surface 111, which reflects light emitted from the light source 1 towards another apparatus (not shown in the drawings) and transmits light from the other apparatus towards the light-receiving elements 2 and 6.
Reference numeral 120 denotes a second prism for guiding light from the other apparatus, which has been transmitted by the beam-splitting surface 111, to the light-receiving element 2 or the light-receiving element 6.
Next, the functionality of the overall free-space optics communication apparatus is explained in accordance with the direction in which the light travels.
First, the light sent out by the light source 1 is approximately collimated by a collimator lens 103, and is incident on the first prism 110. Then, the sent light is reflected by the beam-splitting surface 111 towards a beam expander 140, and emerges from the prism 110 through an incident/emergent port 112. The beam expander 140 broadens the width of the sent light and projects it towards the other apparatus (not shown in the drawings).
On the other hand, the light received from the other apparatus is transmitted by the beam expander 140, is incident on the first prism 110 through the incident/emergent port 112, and is incident on the second prism 120 after being transmitted by the beam-splitting surface 111. A portion of the received light which is incident on the second prism 120 is reflected by a half-mirror surface 122 in the direction of the light-receiving element 2, is condensed by a lens 102, and reaches the light-receiving element 2. The received light which is transmitted through the half-mirror surface 122 emerges after passing through a third prism 130, is condensed by a lens 103, and reaches the light-receiving element 6.
First, the light sent out by the light source 1 is approximately collimated by a collimator lens 101, and is incident on the first prism 110. Then, the sent light is reflected by the beam-splitting surface 111 towards a beam expander 140, and emerges from the prism 110 through an incident/emergent port 112. The beam expander 140 broadens the width of the sent light and projects it towards the other apparatus (not shown in the drawings).
The reason for providing this parallel portion 121 is to prevent that light which is emitted from the light source 1 is incident on the light-receiving element 2. That is to say, by providing the parallel portion 121 with a certain length Z, it is ensured that the light emitted from the light source 1 is not incident on the light-receiving element 2, as shown by the bold broken line 150 in FIG. 5. Thus, it is possible to prevent cross-talk which may be caused by receiving light sent from the light source 1 with the light-receiving element 2.
However, since in the structure shown in FIG. 5 it is necessary to provide the second prism 120 with the parallel portion 121, the second prism 120 becomes large.